Solvent de-asphalting is a process that separates heavy hydrocarbon oil into two phases, an asphalt phase, which contains substances of relatively low hydrogen to carbon ratio often called asphaltene type materials and a de-asphalted oil phase, which contains paraffinic type material substances of relatively high hydrogen to carbon ratio often called De-asphalted Oil (DAO). Therefore, it may be said that solvent de-asphalting is possible because different compounds have different solution affinity for each other and some combination are completely miscible while other combinations are almost immiscible. The ability of the solvent to distinguish between high carbon to hydrogen asphaltene type and low carbon to hydrogen paraffinic type materials is termed as selectivity.
Solvent de-asphalting of heavy residual hydrocarbon oils using solvents to remove contaminants such as asphaltenes, metals and sulphur constituents has long been a standard processing practice in the petroleum refining industry. In the era of high crude oil prices, refiners prefer to process cheaper heavier crude. The large residue generated from heavy crude can be upgraded through solvent de-asphalting process to produce DAO for secondary processes.
Solvent de-asphalting of short residue is primarily being employed for (lube-oil base stocks) LOBS production. However, the process also employed to produce more feedstock for secondary conversion processes such as Fluid Catalytic Cracking (FCC) and hydrocracking so as to upgrade bottom of the barrel and improve distillate yield. Conventionally, Propane de-asphalting is predominantly used for production of LOBS feedstock and slightly heavier paraffinic solvents are used for production of feedstock for conversion process. Propane de-asphalting produces high quality DAO suitable for LOBS production with limited DAO yield while use of heavier solvent say, C5 hydrocarbons results in increased DAO yield at the cost of quality. Thus, the choice of solvent for de-asphalting is made based on the requirement of DAO yield and rejection level of contaminants leading to requirement of two different processing units.
The use of light hydrocarbon to upgrade heavy hydrocarbon oils is the subject of many patents, for instance U.S. Pat. Nos. 4,502,944, 4,747,936, 4,191,639 3,975,396, 3,627,675, and 2,729,589. Use of mixture of propane, CO2, H2S is reported in U.S. Pat. No. 4,191,639 and an increase in DAO yield for same quality is also reported.
Delayed coking is a process used in petroleum refineries to crack petroleum residue, thus converting it into gaseous and liquid product streams and leaving behind solid, carbonaceous petroleum coke. The excess generation of low value petroleum coke in Delayed coking unit causes problems of coke handling and also reduces the profitability. In order to improve the conversion of the heavy residue feedstock, different process configurations employing combination of delayed coking and solvent de-asphalting processes have been employed in the prior art.
U.S. Pat. No. 3,617,481 discloses a combination of De-asphalting-Coking-Hydrotreating processes. The residue feed is first de-asphalted in a de-asphalting extractor and then the asphalt pitch is coked to obtain residual coke, by directly routing to the coking reactor. The metal containing coke is gasified in a gasifier in presence of steam and the said activated coke is employed for hydrotreating.
U.S. Pat. No. 6,673,234 describes a combination of low degree solvent asphalting and delayed coking process. In the first step, a low degree solvent de-asphalting is employed to remove the heavy asphaltene portion of the residue feedstock, in which the yield of de-asphalted oil ranges from 70 to 95 wt % of residue feedstock. In the second step, the de-asphalted oil containing lesser asphaltenes compared to the residue feedstock, along with an optional residue feed, is fed to the delayed coking section of the process. The main objective of the process is to produce premium quality petroleum coke from the residue feedstock.
U.S. Pat. No. 9,296,959 describes the integration of solvent de-asphalting with resid hydroprocessing and delayed coking. First step of this process consist of solvent de-asphalting of residue feedstock to obtain three fractions namely, de-asphalted oil, resin and pitch. The resin steam is subjected to hydrotreating, in which lighter hydrocarbons are generated and recovered. The hydrotreated resin and pitch combine together and is sent to the delayed coking section. In an embodiment, the hydrotreated resin stream is further subjected to solvent extraction to recover lighter material, before being sent to the delayed coking section.
U.S Pat. Application No. 2017/0029720 describes an enhanced solvent de-asphalting delayed coking integrated process, where the de-asphalted oil is routed to the delayed coker unit for coking. In an embodiment, the solvent de-asphalting is carried out in presence of an adsorbent material for removal of poly nuclear aromatics, sulfur and nitrogen compounds.
It is seen that different schemes have been described in the art wherein a combination of solvent de-asphalting and delayed coking processes. But, in none of the schemes, the issue of recycle fraction removal from delayed coking feed is addressed. In the case where pitch after de-asphalting of vacuum residue is routed directly to delayed coker fractionator bottom, the recycle fraction will mix with the pitch. This pitch with recycle fraction when subjected to delayed coking in coke drums, product yield pattern deteriorates in terms of higher coke yield. In case where the fractionator is made to operate at zero recycle, where condensation of heavy material from product vapors entering the fractionator is avoided, the quality of heavier products like Heavy Coker Gas Oil (HCGO) and Coker Fuel Oil (CFO) deteriorates in terms of increasing density, CCR and asphaltene content, impacting the downstream unit operations like hydrocracker. In view of this it is beneficial to have a process scheme in which the quality of HCGO and CFO is not compromised while reducing recycle ratio in an integrated solvent de-asphalting-delayed coking process scheme.